Death is the opposite of suffering because you're not capable of experiencing it anymore. If you're murdered how can you suffer after you're already dead? That doesn't make sense.
For us who desire death it may very well mean the cease of suffering. But for an animal that doesn't want to die, it's an involuntary death and murder - contrary to their desires; sure, there isn't suffering after they are dead, but there surely is suffering in the process of dying, even if it is a 'peaceful one', mainly because it is an involuntary death, one which they do not desire.
The difference between voluntary and involuntary death is key. If we were to take away that difference, I could go around murdering people's pets, grandma's and aunts just because I felt that I put them out of their misery.
The same applies to ants, moths, mosquitoes and to any living beings; my suffering isn't any greater than that of a turtle, for example; the world is poison and life is suffering, but if that turtle desires to live, even if it does only based on natural instincts, then I'm in no position to crush it and end its life.
That's my opinion, anyways. As cynical and pessimistic as I am, I have no right to impose my will unto other beings, and my worldview doesn't justify the cause of more suffering.
Hugs to you, and to everyone
I appreciate the reply.